Heat mat with thermostatic control

ABSTRACT

A heating pad with a thermostatic control circuit coupled to a resistance heating element. The resistance heating element and an acrylic based polymer adhesive are glued between polyester film layers. The film layers and heating element are contained with a plastic sleeve. The thickness and properties of the polyester film and adhesive forms a fuse. The adhesive delaminates the polyester film at excessive temperatures causing the resistance heating element to sever thereby creating an open circuit that halts operation of the heating pad. The thermostatic control circuit includes a hysteresis circuit that compares analog signals across two thin film resistors to provide a control signal to a power controller to selectively vary the power output to the heating element. The thermostatic control circuit is an integrated circuit board disposed within the fused plastic sleeve.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of U.S. patent application Ser. No.15/061,779 entitled ANALOG THERMOSTATIC CONTROL CIRCUIT FOR A HEATINGPAD filed Mar. 4, 2016, now U.S. Pat. No. ______, which is a Divisionalof U.S. patent application Ser. No. 14/326,591 entitled HEAT MAT WITHTHERMOSTATIC CONTROL filed Jul. 9, 2014, now U.S. Pat. No. 9,370,045which claims the benefit under 35 U.S.C. §119(e) from U.S. ProvisionalPatent Application No. 61/938,336 entitled HEAT MAT WITH THERMOSTATICCONTROL filed Feb. 11, 2014.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a heat mat with thermostatic control.The heat mat's thermostatic control is used to regulate the temperatureof the heat mat and increase the temperature automatically whentemperatures fall below a predetermined set point.

2. The Prior Art

Other types of heat mat control are known in the art. For example, heatmat controls may be found in the following: U.S. Pat. No. 2,425,686 toPorter issued on Sep. 22, 1945; and U.S. Pat. No. 2,918,558 issued toEvans on Sep. 19, 1958;

SUMMARY OF THE INVENTION

The invention relates to a thermostatic control for electric heat mats.Heat mats are used in a variety of applications but most heat mats aresold without any type of thermostatic control due to the high cost ofelectronic thermostats. The thermostatic control for electric heat matsprovided by the invention is a low-cost solution that will turn the heatmat on and off at a pre-determined temperatures that are applicationspecific.

The invention comprises an overmolded control circuit, avariable-resistance temperature sensor and a vinyl based heat mat.

The overmolded control circuit contains either a conductive bi-metalthermal switch or low cost electronic printed circuit board as thecontrolling portion of the design.

In one embodiment, the heat mat includes a thermostatic control circuitcoupled between an electrical source and a resistance heating coil. Thethermostatic control circuit includes a temperature sensor, a referencevoltage generating source, a hysteresis circuit and a power controllerfor the resistance heating element. The hysteresis circuit comparesanalog signals from the temperature sensor to internally generatedreference parameters derived from the reference voltage generatingsource to provide a control signal to the power controller thatselectively varies the power output to the resistance heating element.The thermostatic control circuit comprises an inexpensive and small formfactor integrated circuit board that is connected between a mainselectrical source and the resistance heating element.

The temperature sensor is a variable-resistance, low power temperaturesensor. The reference voltage generating source includes a bridgerectifier to convert AC from the electrical source to unregulated DCthat is supplied to the power controller. The reference voltagegenerating source further includes a voltage reference diode to create astable 5 v reference regardless of load, changes in power supply ortemperature that is supplied to the hysteresis circuit.

The hysteresis circuit includes two thin film resistors and thehysteresis circuit compares analog signals from said temperature sensorto internally generated reference parameters derived from said referencevoltage generating source to provide a control signal to said powercontroller. The power controller includes a MOSFET to selectivelycontrol the power to said heating pad. The thermostatic control circuitcomprises an inexpensive and small form factor integrated circuit boardthat is embedded within a plastic overmold housing that is connectedbetween a mains electrical source and said heating pad.

According to a further embodiment, the invention relates to a layeredheating pad and method for manufacturing same. The heating pad includesmultiple layers comprising in order:

a. a plastic sleeve layer made from a 24 gauge poly vinyl chloride;

b. a layer of polyester material;

c. a middle layer containing high resistance metal alloy wire andpolymer liquid adhesive;

d. an additional layer of polyester material;

e. an additional plastic sleeve layer made from a 24 gauge poly vinylchloride.

The metal alloy is copper, nickel or stainless steel. The polyester filmis polyethylene terephthalate (PET) and is clear to allow for visualinspection of said resistance heating element.The polymer liquid adhesive delaminates the PET film at temperaturesabove about 300 degrees F. to operate like a fuse severing theresistance heating wire and halting operation of the heating pad. Otherthermostatic controls, for example, analog controls may be used incombination with the overmold and/or method for manufacturing same.

In an alternate embodiment, the heating pad includes multiple layersconsisting of in order:

a. a plastic sleeve layer made from a 24 gauge poly vinyl chloride;

b. a layer of polyester material;

c. a middle layer containing high resistance metal alloy wire andpolymer liquid adhesive;

d. an additional layer of polyester material;

e. an additional plastic sleeve layer made from a 24 gauge poly vinylchloride.

The polymer adhesive and said polyester films form a fuse, wherein saidadhesive delaminates the polyester films at temperatures above about300° F. causing the resistance heating element to sever causing an opencircuit that halts operation of the heating pad. The polymer adhesivecomprises a 1.0 mil thick layer of cured adhesive which has a coatingweight of about 16 lbs/3,000 ft², a loop tack of about 4.4 lbs/in; a180° peel adhesion of about 4.3 lbs/in utilizing a 15 minute dwell, ashear adhesion of 24+ hours utilizing ½ in×½ in×500 grams testconditions, and a plasticity of about 2.4 mm.

The heating pad is claimed in combination with an acrylic polymerdissolved in a solvent which cures to form said polymer adhesive. Thesolvent is one of toluene, heptane, isopropanol, acetone, ethanol andcombinations thereof.

The acrylic polymer is dissolved in the solvent to provide aself-crosslinking polymer liquid adhesive having a viscosity of between2,000 and 5,000 cps and a density of between 6 and 8 lbs/gal. Theheating pad is claimed in combination with a degassing station thatdegasses the heating pad under vacuum to (i) remove air and solvent fromwithin the mat and cure the liquid adhesive, and (ii) heat and fuse thepolyvinyl chloride sheets together around the periphery the mat.

According to a further embodiment of the invention, there is provided amethod for manufacturing a heating pad beginning with stringing aresistance heating wire out on a form in a pattern. The patterned wireand form is placed on a first polyester film. A liquid adhesive isprovided comprising a single-package, self-crosslinking polymer having aviscosity of between 2,000 and 5,000 cps and a density of between 6 and8 lbs/gal. The adhesive is coated on to a second polyester film. Thefirst and second polyester films are then glued together to encase thepatterned wires therebetween.

The polyester encased wires are then removed from the form to create anintermediate mat. The mat is sandwiched between two larger polyvinylchloride sheets that extend beyond the mat's periphery to form anensemble.

The liquid adhesive is cured and degassing under vacuum to reduceentrapped air from within the mat. At the same time the mat is heated tofuse the two layers of polyvinyl chloride together around the peripheryof the mat to form the heating pad.

The polyester film is a polyethylene terephthalate (PET) film. Theliquid adhesive is an acrylic polymer dissolved in a solvent. Thesolvent includes toluene, heptane, isopropanol, acetone, ethanol andcombinations thereof. A 1.0 mil thick layer of cured adhesive has acoating weight of about 16 lbs/3,000 ft², a loop tack of about 4.4lbs/in; a 180° peel adhesion of about 4.3 lbs/in utilizing a 15 minutedwell, a shear adhesion of 24+ hours utilizing ½ in×½ in×500 grams testconditions, and a plasticity of about 2.4 mm. The mat comprises a fusethat delaminates the polyimide electric thin film at high temperaturesabove after 300° F. causing the resistance heating wire to burn upcausing an open circuit that halts operation of the heating pad.

BRIEF DESCRIPTION OF THE DRAWINGS

The advantages, nature, and various additional features of the inventionwill appear more fully upon consideration of the illustrativeembodiments now to be described in detail in connection withaccompanying drawings. In the drawings wherein like reference numeralsdenote similar components throughout the views:

FIG. 1 is a schematic view of the first embodiment of the invention withan overmolded electronic PCB thermostatic control in the 120 v poweradapter.

FIG. 2 is a schematic view of the second embodiment of the inventionwith an electronic PCB thermostatic control located in the connectionsleeve of the electric heat mat.

FIG. 3 is a schematic view of the third embodiment of the inventionshowing a bi-metal thermal switch thermostatic control located in theconnection sleeve of the electric heat mat.

FIG. 4 is a schematic view of the thermostatic control circuit.

FIG. 5 is a circuit diagram of the thermostatic control circuit.

FIG. 6 is a flowchart showing the steps for manufacturing a heating pad.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention relates to a thermostatic control and encapsulatingmaterial for a heating pad, and more specifically a waterproof heatingpad for indoor and outdoor use. Conventional thermostats have not beenwidely adopted for use in waterproof heating pads, or heating padsdesigned for indoor and outdoor use. Such thermostats have been eithertoo expensive or too bulky or both. Accordingly, the primary purpose ofthe invention is to provide an inexpensive and small form factorthermostat. The thermostat is encapsulated in an overmolded housingwhich protects it from intrusion from liquids.

The thermostat is provided in printed circuit board (pcb) form, withcontacts at the periphery for soldered connections to a power supply,temperature sensor and resistance heating coil. The pcb is inherently aflat thin panel, having dimensions on the order of an inch wide, severalinches long and a fraction of an inch thick. Accordingly, this panelgeometry is well suited for use in a heating pad that will be used as amat.

In one embodiment a heating pad assembly 10 is shown in FIG. 1 with aplug 12 and temperature sensor 16 coupled to a heating pad 18. Plug 12includes prongs 12 a for connecting to electrical mains, for example120V AC. Prongs 12 a are supported in a plug housing 12 b joined to aplug cable 12 c that couples to a connection sleeve 18 c on one side ofheating pad 18. A temperature sensor 16 joins to a sensor cable 16 cthat also couples to the connection sleeve 18 c. Within connectionsleeve 18 c, plug cable 12 c and sensor cable 16 c and resistanceheating element 18 a are operatively coupled together. A thermostat pcb14 is located within plug housing 12 b. For example, prongs 12 a, pcb 14and the end of plug cable 12 c are soldered together and placed within amold. Plug housing 12 b is then overmolded around pcb 14 to seal it andits soldered connections in a waterproof shell.

In a further embodiment a heating pad assembly 30 is shown in FIG. 2with a plug 32 and temperature sensor 36 coupled to a heating pad 38.Plug 32 includes prongs 32 a for connecting to electrical mains, forexample 120V AC. Prongs 32 a are supported in a plug housing 32 b joinedto a plug cable 32 c that couples to a connection sleeve 38 c on oneside of heating pad 38. A temperature sensor 36 joins to a sensor cable36 c that also couples to the connection sleeve 38 c. Within connectionsleeve 38 c, plug cable 32 c and sensor cable 36 c and resistanceheating element 38 a are operatively coupled together. A thermostat pcb34 is sealed within an overmold and located within connection sleeve 38c. The connection sleeve 38 c is attached on, formed from, or sandwichedbetween two layers of durable thermoset material forming the outerprotective layer of the heating pad. The thermoset material may bepolyvinyl chloride (pvc) in 24 gauge thickness or similar material.

The resistance heating element 38 a is encased within inner protectivelayers of polymer sheet material, for example, a thermoplastic materiallike polyester or more specifically polyethylene terephthalate (PET).The inner protective layers are glued to each other with the resistanceheating element contained therein. The glue is a polymer based adhesivehaving a viscosity of between 2,000 and 5,000 cps and a density ofbetween 6 and 8 lbs/gal.

In practical tests, an adhesive meeting safety and waterproofrequirements has properties which a 1.0 mil thick layer of curedadhesive has a coating weight of about 14 to 18 lbs/3,000ft², ideally 16lbs/3,000 ft², a loop tack of about 4.0 to 5.0 lbs/in, ideally 4.4lbs/in; a 180° peel adhesion of about 4.0 to 4.6 lbs/in, ideally about4.3 lbs/in utilizing a 15 minute dwell, a shear adhesion of 24+ hoursutilizing ½ in×½ in×500 grams test conditions, and a plasticity of about2 to 3 mm, ideally 2.4 mm.

Chemically the liquid adhesive is polymer, for example, an acrylicpolymer dissolved in a solvent. Suitable solvents include toluene,heptane, isopropanol, acetone, ethanol and combinations thereof. Incertain instances the solvent comprises a solvent blend including 2 ormore, 3 or more, 4 or more or all of toluene, heptane, isopropanol,acetone, and ethanol. One adhesive meeting the above requirement isAshland Aroset 390M. Aroset is a single-package, self-crosslinkingacrylic polymer that cures at moderate temperatures upon completesolvent removable. Once cured, the polymer is a pressure sensitiveadhesive. The vacuum degassing will apply sufficient pressure allow theadhesive to securely bond the two PET films together.

The resistance heating element and crosslinked acrylic polymer and innerPET sleeve operate like a fuse. Upon overheating, the adhesive expandsand delaminates the PET film at high temperatures, for example aboveabout 300° F. The as the thin film separates it tears the resistanceheating wire. Once the heating wire is severed an open circuit shutsdown the operation of the heating pad. More particularly, the MOSFETpower controller will shut down if the resistance wire is no longercompleting a circuit back to the bridge rectifier.

In yet another embodiment a heating pad assembly 50 is shown in FIG. 3with a plug 52 and temperature sensor 56 coupled to a heating pad 58.Plug 52 includes prongs 52 a for connecting to electrical mains, forexample 120V AC. Prongs 52 a are supported in a plug housing 52 b joinedto a plug cable 52 c that couples to a connection sleeve 58 c on oneside of heating pad 58. A temperature sensor 56 joins to a sensor cable56 c that also couples to the connection sleeve 58 c. Within connectionsleeve 58 c, plug cable 52 c and sensor cable 56 c and resistanceheating element 58 a are operatively coupled together. A bi-metallicthermostat 54, for example a bi-metal thermal switch, is overmolded andlocated within connection sleeve 38 c.

To create the overmold, the pcb is suspended centrally a molding cavity.A molten thermoplastic material resin is injected into the cavity toencapsulate the pcb. This process is effective in protecting theinternal electrical components and may be utilized in combination with abi-metallic thermostat.

The thermostat is an electronic circuit that monitors the temperature,establishes a temperature threshold and controls the power output to theresistance coil. A temperature sensor is provided, for example, a lowpower temperature sensor. In one embodiment the temperature sensor isdisposed at the end of sensor cable, so it can be placed in varyingdistance to the heating elements. The thermostatic control circuitutilizes a voltage reference and hysteresis circuit to set a temperaturethreshold for the resistance heating coil. The control circuit thenvaries the power provided to the resistance heating coil to achieve thedesired temperature at the sensor.

The thermostatic control circuit 70 is formed on a printed circuit board(pcb), an electrical schematic of which is shown in FIG. 4. On the inputside, an electrical mains 72 is coupled to the control circuit, and onthe output side, the control circuit is coupled to the resistanceheating element 74. A further input is received from avariable-resistance, low power temperature sensor 76.

More specifically, electrical mains 72 provides a 120V AC input 72 a toreference voltage generating source 80. Reference voltage generatingsource 80 provides a high voltage DC output 80 a to power controller100. Reference voltage generating source 80 also provides low voltage DCoutput 80 b to hysteresis circuit 90. More particularly, low voltage DCoutput 80 b is a 5V DC reference voltage that remains stable regardlessof load, changes in power supply or temperature. Reference voltagegenerating source 80 also provides low voltage DC output 80 b totemperature sensor 76. The low voltage DC output to temperature sensor76 may be the same output or a different output than provided tohysteresis circuit 90, as illustrated by the two dotted lines.Temperature sensor 76 is a variable-resistance, low voltage temperaturesensor that generates an analog signal 76 a that is provided tohysteresis circuit 90.

In using thermostatic control circuit pcb 14,70 in the Embodiment ofFIG. 1, the pcb is disposed within plug housing 12 b. Prongs 12 aprovide AC input 72 a. Prongs 12 a, pcb 14, 70 and the end of plug cable12 c are place into a mold, and plug housing 12 b is overmolded to sealand secure the components together. Low voltage DC output 80 b issupplied along plug cable 12 c, through connection sleeve 18 c, thenalong sensor cable 16 c to power sensor 16. Analog signals 76 a fromsensor 76 are supplied along sensor cable 16 c, through connectionsleeve 18 c, then along plug cable 12 c to hysteresis circuit 90 withinpcb 14, 70. Power output 100 a is supplied along plug cable 12 c,through connection sleeve 18 c to resistance heating element 18 a, 74.The connection sleeve is formed by placing the ends of plug cable 12 cand sensor cable 16 c in a mold and overmolding the connection sleeve asa flat pack. The strain relief shown off the left side of connectionsleeve 18 c may be formed integrally with the overmold.

In using thermostatic control circuit pcb 34, 70 in the Embodiment ofFIG. 2, the pcb is connected to plug cable 32 c and sensor cable 36 c.This pcb is then placed in a mold and the connection sleeve isovermolded as a flat pack. The strain relief shown off the left side ofconnection sleeve 38 c may be formed integrally with the overmold. ACinput 72 a is provided along plug cable 32 c. A low voltage DC output 80b and analog signals 76 a are provided along sensor cable 36 c. Theovermolded connection sleeve is secured to heating pad 38 and poweroutput 100 a is connected to resistance heating element 38 a.

In a further embodiment shown in FIG. 3, a bi-metal thermostat 54 isconnected to plug cable 52 c and sensor cable 56 c. The bi-metalthermostat 54 is then placed in a mold and the connection sleeve isovermolded as a flat pack. The strain relief shown off the left side ofconnection sleeve 58 c may be formed integrally with the overmold. ACinput is provided along plug cable 52 c.

Thermostatic control circuit 70 is configured as a temperature thresholdsetting device. In one embodiment shown in FIG. 5 the reference voltagegenerating source 80 includes a bridge rectifier 80 c and a voltagereference diode 80 d. The bridge rectifier converts the inputted AC tounregulated high voltage DC output 80 a. Voltage reference diode 80 dcreates a stable voltage reference regardless of load, or changes inpower supply or temperature. The voltage reference is 5 v, for example,output via low voltage DC output 80 b.

Hysteresis circuit 90 includes two thin film resistors 90 r. Sensor 76includes a variable resistor 76 v that is supplied with low voltage DCoutput 80 e. The variable resistor uses electrical impulses to measurethe temperature of the heat pad using parameters created with thehysteresis circuit to set the temperature threshold for the resistanceheat coil. By adjusting the variable resistor, different temperaturethresholds can be set. The hysteresis circuit 90 utilizes the lowvoltage DC output 80 b and sensor output 76 a to supply a control signal90 to power controller 100.

Once the temperature threshold is acquired, a MOSFET 100 m within powercontroller is utilized to selectively control the power that isoutputted to the resistance heating coil. The thermostatic controlcomprises an inexpensive and small form factor integrated circuit boardthat is embedded within a plastic overmold housing that is connectedbetween a mains electrical source and said heating pad. For high heat orcommercial applications larger MOSFET power controllers may be utilized.The pcb containing such MOSFETs may require passive cooling. One type ofcooling device includes a heat sink made from metal or other material.For example, the pcb or MOSFET may be conductively coupled to analuminum plate. The plate may be structured as a sub-layer formedbeneath the pcb where it is electrically insulated and thermally coupledto same.

The thermostatic control encased within an overmold flat pack and theresistance heating coil are sealed within multiple layers comprising inorder

a. a plastic sleeve layer made from a 24 gauge poly vinyl chloride

b. a layer of thermoplastic material comprising PET

c. a layer of polymer liquid adhesive

d. a layer containing high resistance metal alloy wire

e. an additional layer of thermoplastic material comprising PET

f. an additional plastic sleeve layer made from a 24 gauge poly vinylchloride.

A method of manufacturing a heating pad, according to a furtherembodiment of the invention, will now be described with respect to FIG.6. In summary, a polymer adhesive having specific chemical andrheological properties is used to encase a resistance heating wirebetween two thermoplastic polyester sheets to produce a mat. The mat isthen sandwiched between two layers of PVC. In the event a section of theresistance heating wire overheats, the adhesive expands causing thepolyester sheets to delaminate thereby severing the wire. The severedwire creates an open circuit that halts operation of the heating pad.

The manufacturing method begins with stringing 202 a resistance heatingwire out on a form. The form may be configured as a board with shortpegs laid out in a pattern. The wire is wrapped around the pegs takingthe shape of the pattern, for example, a sinusoidal wave pattern, toroute the wire for even heating across the entire surface of the heatingpad. The pegs may be withdrawn down into the board when the wire isready to be removed from the form. The form and wrapped wires are placedon top a first polyester or PET film 204.

A liquid adhesive is provided 206 comprising a single-package,self-crosslinking acrylic based polymer having a viscosity of between2,000 and 5,000 cps and a density of between 6 and 8 lbs/gal. Additionaladhesive properties include one or more of: a 1.0 mil thick layer ofcured adhesive has a coating weight of about 16 lbs/3,000 ft2, a looptack of about 4.4 lbs/in; a 180° peel adhesion of about 4.3 lbs/inutilizing a 15 minute dwell, a shear adhesion of 24+ hours utilizing ½in×½ in×500 grams test conditions, and a plasticity of about 2.4 mm. Theadhesive is dissolved in a solvent selected from the group consisting oftoluene, heptane, isopropanol, acetone, ethanol and combinationsthereof.

In practical applications, Aroset 390M self-crosslinking pressuresensitive adhesive available from Ashland has been used. Aroset 390M isa single-package, self-crosslinking acrylic polymer that cures atmoderate temperatures upon complete solvent removal. At roomtemperature, full cure may take up to one week. In use, Aroset 390Mfunctions as a pressure sensitive adhesive. The adhesive is dissolved ina solvent blend including toluene, heptane, isopropanol, acetone, andethanol.

A layer of adhesive is coated 208 on to a second polyester or PET filmthat is placed on top of the patterned wire. The two PET film layers areglued 210 together encasing the wire therebetween. Now that the wire isheld in place by the adhesive and PET film layers, the wire can beremoved 212 from the form. For example, the pegs are withdrawn, allowingthe resistance wire to come free of the form and remain adhered to theadhesive and two polyester sheets in the formed or patterned shape toform an intermediate mat.

The thermoplastic films that may be used to form intermediate mat may bebetween 1.8 and 2.2 mils thick with a density between 1.2 and 1.6 g/cm³,ideally 2 mils thick and 1.4 g/cm³. Such films would further have atensile in the range of 17 to 25 psi, ideally 21 psi. The films wouldhave a strength between 20 and 30 Kg/mm², ideally 24 Kg/mm². The filmelongation would be 160 to 240%, ideally 200%. The film would possess anAt Break value between 100 and 140, ideally 120. The friction valuewould be between 0.3 and 0.6 μk, ideally 0.4 82 k. The coefficient valuewould be between 0.3 and 0.7 μs, ideally 0.5 μs. The surface roughnesswould be 0.013 Ra, 0.12 Rz and 0.25 Rmax. The optical haze would beabout 1.4%, the light transmission about 93% and the gloss about 200%.The thermal heat value would be in the range of 1.2 to 1.6% MD measuredby the SKC method, ideally 1.4%. The thermal shrinkage value would be inthe range of 0.3 to 0.7 TD measured at 150 degrees C.×30 mins, ideally0.5 TD.

Next, the mat is sandwiched 214 between two layers of larger polyvinylchloride sheets to form the heating pad. Since the PVC sheets are largerthan the mat they can be fused together. The heating pad are then placedwithin a clamshell, subject to vacuum and heated to cure the adhesiveand fuse the edges of the PVC to each other. In other words theassembled pad is degassing under vacuum to reduce entrapped air andevaporated adhesive solvent from within the mat. Degassing includessubjecting the sealed clamshell to −20 to −35 inches Hg vacuum, ideallybetween −26 to −30 inches Hg vacuum. Heating includes placing theclamshell within an oven for 0.5 to 2.0 hours at 300 to 400 degrees F.,ideally about 1.3 hours at 350 degrees F.

The thermostatic control circuit described above and in FIGS. 4 and 5 isalso sandwiched between two layers of PVC to form a flat pack. The flatpack is coupled to the leads of the resistance heating wire. In theembodiment of FIG. 2, the flat pack is placed adjacent to the mat toform the connection sleeve 38 c. The flat pack and mat are thencollectively sandwiched on or between two layers of PVC, which forms thesealed exterior of the heating pad.

The mat comprises a fuse. In use, an overheat condition causes theadhesive to expand and delaminate the PET film at high temperaturesabove about 300° F. causing the resistance heating wire to severecausing an open circuit that halts operation of the heating pad.

Having described preferred embodiments for (which are intended to beillustrative and not limiting), it is noted that modifications andvariations can be made by persons skilled in the art in light of theabove teachings. The power source designated as 120V AC could be 220 orhigher for commercial applications. The mat could be manufactured fromequivalent materials or other processing steps known within theindustry. The circuit blocks or components could include equivalentdevices. It is therefore to be understood that changes may be made inthe particular embodiments of the invention disclosed which are withinthe scope and spirit of the invention as outlined by the appendedclaims. Having thus described the invention with the details andparticularity required by the patent laws, what is claimed and desiredprotected by Letters Patent is set forth in the appended claims.

What is claimed is:
 1. A heating pad comprising: a resistance heatingelement coupled to a thermostatic control circuit, wherein saidresistance heating element is sealed within multiple layers comprisingin order: a plastic sleeve layer made from polyvinyl chloride; a layerof polyester film; a middle layer containing said resistance heatingelement comprising high resistance metal alloy wire and a polymeradhesive; an additional layer of polyester film; and an additionalplastic sleeve layer made from polyvinyl chloride.
 2. The heating pad ofclaim 1, wherein said adhesive comprises a solvent-evaporated curedacrylic based crosslinked polymer adhesive having a loop tack of about4.4 lbs/in, and a 180° peel adhesion of about 4.3 lbs/in utilizing a 15minute dwell.
 3. The heating pad of claim 2, wherein each of saidpolyester film layers comprises a polyethylene terephthalate (PET) filmbetween about 1.8 and 2.2 mils thick, wherein said PET film has athermal heat value in the range of about 1.2 to 1.6% MD measured by theSKC method, and a thermal shrinkage value between about 0.3 and 0.7 TDmeasured at 150 degrees C.×30 minutes.
 4. The heating pad of claim 3,wherein said polymer adhesive and said polyester film layers comprise afuse, wherein said adhesive delaminates the polyester film layers attemperatures above about 300° F. causing the resistance heating elementto sever thereby creating an open circuit that halts operation of theheating pad.
 5. The heating pad of claim 4, wherein said thermostaticcontrol circuit includes a reference voltage generating source and apower controller that are connected in a loop with said resistanceheating element, whereby the open circuit severs the loop connection. 6.The heating pad of claim 5, wherein said metal alloy wire includes amaterial selected from the group consisting of copper, nickel, stainlesssteel and combinations thereof.
 7. The heating pad of claim 6, whereinsaid PET film has a light transmission of about 93% to allow for visualinspection of said resistance heating element.
 8. The heating pad ofclaim 7, wherein said thermostatic control circuit further includes ahysteresis circuit and a variable-resistance, low power temperaturesensor coupled to said hysteresis circuit, wherein said hysteresiscircuit provides a control signal for said power controller toselectively vary the power output to said resistance heating elementbased on temperature sensor data.
 9. The heating pad of claim 8, whereinsaid hysteresis circuit includes a first and second thin film resistor,wherein said hysteresis circuit compares an analog signal derived fromsaid temperature sensor across said first thin film resistor to ananalog signal derived from said reference voltage generating sourceacross said second thin film resistor to provide the control signal. 10.The heating pad of claim 9, wherein said thermostatic control circuitcomprises a small form factor integrated circuit board that is embeddedwithin a plastic housing that is connected between a mains electricalsource and said resistance heating element.
 11. The heating pad of claim10, wherein said integrated circuit board containing said thermostaticcontrol circuit is disposed within an overmolded power plug.
 12. Theheating pad of claim 4, wherein said adhesive is selected from the groupconsisting of: a toluene solvent-evaporated cured acrylic basedcrosslinked polymer adhesive; a heptane solvent-evaporated cured acrylicbased crosslinked polymer adhesive; an isopropanol solvent-evaporatedcured acrylic based crosslinked polymer adhesive; an acetonesolvent-evaporated cured acrylic based crosslinked polymer adhesive; andan ethanol solvent-evaporated cured acrylic based crosslinked polymeradhesive.
 13. The heating pad of claim 5, wherein said adhesive is atoluene, heptane, isopropanol, acetone, ethanol blend solvent-evaporatedcured acrylic based crosslinked polymer adhesive.
 14. The heating pad ofclaim 13, wherein said reference voltage generating source includes abridge rectifier to convert AC from the mains electrical source tounregulated DC that is supplied to said power controller.
 15. Theheating pad of claim 5, wherein said adhesive is an air andsolvent-degassed cured acrylic based crosslinked polymer adhesive, andwherein said polyester film layers have a density between about 1.2 and1.6 g/cm³, elongation between about 160 to 240% and at break valuesbetween about 100 and
 140. 16. The heating pad of claim 6, wherein saidpolymer adhesive has a shear adhesion of 24+ hours utilizing ½ in×½in×500 grams test conditions and a plasticity of about 2.4 mm.
 17. Theheating pad of claim 10, wherein said plastic sleeve layer and saidadditional plastic sleeve layer are both made from 24 gauge polyvinylchloride (PVC), wherein said sleeve layers are heat fused togetheraround the periphery of the pad.
 18. The heating pad of claim 17,wherein said reference voltage generating source includes a voltagereference diode to create a stable 5 v reference voltage regardless ofload, changes in power supply or temperature, wherein said stable 5 vreference voltage is supplied to said hysteresis circuit.
 19. Theheating pad of claim 18, wherein said power controller comprises aMOSFET.
 20. The heating pad of claim 19, wherein said integrated circuitboard containing said thermostatic control circuit is disposed withinsaid fused PVC sleeve layers.